Android Button findViewByID返回null

Question

我的主表单上有三个按钮。我加了第三个。当我尝试使用findViewById分配按钮时，返回null。

这是我的main.xml

<?xml version="1.0" encoding="utf-8"?>
<TableLayout
   xmlns:android="http://schemas.android.com/apk/res/android"
   android:orientation="vertical"
   android:layout_width="match_parent"
   android:layout_height="match_parent" android:weightSum="1" android:clickable="false">
   <TextView android:layout_width="wrap_content"
        style="@style/TitleText"
        android:text="@string/app_name" 
        android:background="@drawable/black_gradient">
   </TextView>
   <Button android:id="@+id/btnvalidatemark"
        android:background="@drawable/grad_btn_green"
        style="@style/ButtonText" android:text="@string/validate_mark_button">
   </Button>
   <Button android:id="@+id/btncheckregistry"
        android:background="@drawable/grad_btn_yellow"
        style="@style/ButtonText" android:text="@string/check_registry_button">
    </Button>
    <Button android:id="@+id/btnregcreds"
        android:background="@drawable/grad_btn_red"
        style="@style/ButtonText" android:text="@string/registry_config_button">
   </Button>
</TableLayout>

这是我的活动代码：

package uid.android.uidchecker;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.view.Window;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.util.Log;

public class UIDCheckerActivity
        extends Activity
{

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        setContentView(R.layout.main);
        Button validatebutton = (Button) findViewById(R.id.btnvalidatemark);
        Button checkbutton = (Button) findViewById(R.id.btncheckregistry);
        Button setcredbutton = (Button) findViewById(R.id.btnregcreds);
        validatebutton.setOnClickListener(new OnClickListener()
            {

                public void onClick(View v)
                {
                    Intent i = new Intent(UIDCheckerActivity.this, SelectFileActivity.class);
                    i.putExtra("func", "validatemark");
                    startActivity(i);
                }
            });
        checkbutton.setOnClickListener(new OnClickListener()
            {

                public void onClick(View v)
                {
                    //Intent i = new Intent(UIDCheckerActivity.this, validatemark.class);
                    Intent i = new Intent(UIDCheckerActivity.this, SelectFileActivity.class);
                    i.putExtra("func", "checkregisrty");
                    startActivity(i);
                }

            });
        setcredbutton.setOnClickListener(new OnClickListener()
            {

                public void onClick(View v)
                {
                    Intent i = new Intent(UIDCheckerActivity.this, RegistryCredsActivity.class);
                    startActivity(i);
                }

            });
    }

}